1. Field of the Invention
The present invention relates to a carrier for an electrophotographic developer and a process of producing the same. More particularly, it relates to a carrier which, when applied to full color electrophotographic developers, achieves excellent image characteristics and extended service life and to a process of producing the carrier.
2. Description of the Related Art
Electrophotography comprises the steps of charging and imagewise exposing a photoreceptor to form an electrostatic latent image thereon, developing the latent image with a developer containing a toner, and transferring and fixing the toner image onto a recording medium. The developer includes a two-component developer comprising a toner and a carrier and a one-component developer such as a magnetic toner.
A two-component developer containing a carrier is widely used as a full color developer or a developer for high-speed developing apparatus by virtue of its advantages such as excellent image quality.
Full color developers which have recently enjoyed an increasing demand are required to rapidly charge a supplied toner and to have capability of continuous development over a broad recording area. Further, advanced electrophotographic recording equipment is getting more compact with a smaller developing sleeve diameter, and the amount of the developer to be loaded has been reduced. These trends have boosted the demand for a carrier for the developer to have improved charging capabilities, an extended service life, and capability of realizing high image quality.
Under these circumstances, a carrier to be used is required to have toner holding capability, toner charging capability, and a reduced particle size for making a softer magnetic brush. Carrier scattering is a constant problem that accompanies size reduction of carrier particles, and a number of countermeasures against this have been proposed.
JP-A-9-197721 proposes a carrier that does not cause an image defect due to carrier adhesion even in high-speed development and a developer containing the carrier. In the proposal, the size of primary particles of a raw material is specified in terms of number average primary particle diameter (Dv) and a volume average primary particle diameter (Dn) to achieve uniformization of magnetization in an attempt to solve the carrier scattering problem. However, it turned out impossible to prevent scattering of small-diameter ferrite particles having an average particle size of 20 to 45 μm even where the Dv/Dn ratio falls within the range of from 1.0 to 2.0 as specified.
The carrier core particles tested in Examples of JP-A-9-197721 supra have an average particle size of 65 μm. It appears that the contemplated effects are little exerted on such carrier particles as small as 20 to 45 μm that scatter easily. It is also assumed that average size reduction of carrier particles requires, of necessity, size reduction of the raw material.
A number of proposals have also been made with regard to magnetic characteristics or particle size distribution of a carrier for being held in a magnetic brush.
For example, JP-A-2001-27828 discloses a carrier which has a weight average particle size of 35 to 55 μm, contains 0 to 15% of particles smaller than 22 μm and 0 to 5% of particles greater than 88 μm, has a specific resin coat, and exhibits a magnetization of 70 to 120 emu/g in a magnetic field of 1 KOe. A carrier having a higher magnetization is admittedly show a wider margin against scattering but, in turn, forms a harder magnetic brush, which will make it difficult to achieve high-quality soft development.
A carrier having a reduced content of particles in the smaller size region of size distribution tends to show better results in connection with the carrier scattering problem, as have been suggested in many reports. However, there are limits in this regard from the technical aspect (e.g., limits of classifying technique and yield) and the economical aspect.
A large number of proposals have been made with respect to small-diameter carriers. Nevertheless, mere application of techniques on conventional ferrite carriers having an average particle size of 60 μm or greater to ferrite carriers having an average particle size of 20 to 45 μm fails to sufficiently settle the carrier scattering problem.